1. Technical Field
This invention relates to a prosthesis for use in joint replacement. More particularly, the present invention relates to a modular prosthesis for replacement of the head portion of the femur, which may be adapted to other joints such as the elongated bone of the shoulder.
2. Background Art
There are various diseases and injuries affecting joints that cause restriction of the joint, loss of movement and pain. Arthroplasty is the surgical treatment of these disorders and aims at restoration of movement at the joint.
Previously, prosthesis components, and particularly femoral prosthesis components that are utilized for surgical reconstruction of a human hip joint, have incorporated relatively stiff intramedullary fixation stems. These stems are fabricated of suitable, biocompatible metallic alloys and generally have integral neck and head portions. Because the stems of these components are stiff, they do not provide significant flexure along their length. However, the surrounding bone within which the stem is implanted is somewhat flexible. Therefore, a stiff stem, relative to the more flexible structure of the bone, results in a composite structure wherein the flexural rigidity of the constituent parts varies significantly.
The use of relatively stiff intramedullary stems has been clinically suspected of producing adverse and destructive bone reactions over a long period of time. Conventional relatively stiff stems reduce the forces distributed to surrounding bone to levels significantly below normal anatomical levels of an intact femur. More particularly, stiff stems can also be attributed to the development of reduced levels of force or stress shielding within the surrounding support of bone structure. In addition, stiff stems can be attributed to producing micromotion at the stem and bone interface. Both the presence of interface micromotion and the reduced stresses on the bone can result in adverse bone reactions which have been attributed to the diminution of bone mass at the interface and also within the surrounding bone matrix. Understandably, loss of bone is detrimental to the function of the implant and can produce loosening of the prosthesis and accompanying loss of articular joint or hip function. Therefore, under the influence of reduced levels of bone stress distribution incident to stiff conventional stems, adverse bone reaction may occur postoperatively where the adjacent bone structure degenerates, diminishes or atrophies. This resultant bone loss can seriously affect the structural integrity of the adjacent supportive bone and may ultimately lead to significant loss or compromise of the long-term function of the implant prosthesis if the resulting pain and/or loss of function becomes significantly intolerable to the patient. Depending upon the severity of these functional factors, surgical revision may be indicated.
Since femoral stem stiffness is the major cause of bone resorption or atrophy, a more flexible stem would be desirable. However, a more flexible femoral stem would have to provide adequate strength to endure the stresses of the body. A variety of ways have been identified to increase stem flexibility.
Geometric changes have been employed to reduce femoral stem stiffness, such as flutes, slots or hollowed cores. A problem with flutes, slots and hollow cores is that while they impart increased flexibility, they decrease the strength of the prosthesis because they require cutting away of some of the supporting structure of the stem.
In addition to researching new geometries to reduce femoral stiffness, there has been research into developing some composite material that would provide both flexibility and strength. However, these efforts have not been very successful. Much work has been done in the area of using biocompatible alloys. However, these biocompatible alloys have insufficient shearing strength, insufficient resistance to impact and an insufficient endurance limit. The possibility of improving the strength of these biocompatible alloys appears limited. Biocompatible metals with relatively increased flexibility, such as titanium have been used. However, these materials have proved brittle and not strong enough to withstand the stresses of the body. An additional problem with titanium is that it has a notch sensitivity such that it requires careful engineering and design in particular when a porous coating to allow bone ingrowth is applied. The notch sensitivity is an indication of the extent to which the endurance of metals, as determined on smooth and polished specimens, is reduced by surface discontinuities such as tool marks, notches and changes in section. Notch sensitivity increases with hardness and endurance limit. Therefore, titanium is more susceptible to failure if its surface is not smooth.
Conventional prostheses for the replacement of the head portion of the femur are also generally unitary structures. The conventional prosthesis generally includes a stem portion that is designed to extend into the intramedullary cavity within the femur. The stem portion may be secured within the femur by the use of bone cement or other adaptations. The conventional hip prosthesis includes a stem and an integral head portion as mentioned above. The head portion is designed to fit into a joint socket. Since the prosthesis is all one piece the unit is normally entirely of the same materials. This has limited the types of materials used since the prostheses has to be both biocompatible and strong.
In addition, because conventional prostheses are generally unitary devices, the practice has been to maintain a large inventory of differently sized units to accommodate the different sizes of bones. Generally, the patient is evaluated by x-ray or some other means to determine the approximate bone size. Then a prosthesis range is estimated. During the replacement operation, several prostheses within the estimated range are made available, as suggested by the evaluation. The appropriate prosthesis is then selected for insertion into the patient at the time of operation.
Therefore, what is needed is a prosthesis which provides reduced femoral stem stiffness, strength sufficient to withstand the stresses of the weight of the body to avoid failure or permanent deformation, biocompatibility and a good fit to the patient.